The Mogami (最上氏) were descended from the Shiba Clan which in turn descended from the original Ashikaga family(different to the Ashikaga Clan ingame). They were founded by Shiba Iekane's son Kaneyori; the Mogami increased their influence during the Sengoku Period and reached their peak after the Battle of Sekigahara fighting along side Oda Nobunaga.

In 1542, a war broke out between Date Tanemune and his son Date Harumune. This is depicted as the Date Clan's first mission: defeat Tanemune's rebel army. The Date clan's power was diminished by this war (Tenbun war/天文の乱). The Clan Leader, Yoshimori, seized this chance to succeed in getting independence from Date. This is why in game the Date and Mogami start at war. The Mogami were one of the Clans Date Masamune consolidated into a northern alliance against the Sengoku period warlords.

Mogami and Date supported Tokugawa Ieyasu at the famous battle of Sekigahara, after which Mogami's domain was expanded. The clans were given 520,000 koku respectively in return for their loyal services.